Life Mushrooms are items that appear in Super Mario Galaxy and in Super Mario Galaxy 2. They give Mario three extra life bars but if his health drops below four, they will disappear and his health meter will return to normal.
Mario does not gain extra health if he is already affected by a Life Mushroom. Upon grabbing a second one, his health will restore to 6. In Super Mario Galaxy 2, if Mario grabs a Life Mushroom while he has 6 health points, it will give him a 1-Up.